tire size

[Ron Gibson]

Everybody seems to in agreement that both tires need to be the same size. That being said, what is the best way to measure? All of the articles on gear ratios (per Hot Rod, etc) say to measure from the ground to the center of the axle times Pi for the actual rollout distance. Nascar changes stagger between rears with as little as 1/2 lb of air. We don't have the option of going through numerous tires to find two that measure the same size. It seems to me that with the tire loaded, you would want to make the ground to axle center measurement the same by adjusting air pressure (within limits)as opposed to diameter by tape measure.

[John Beckett]

Ron
 
 Find one of the Circle Track parts houses on the internet and they will have a special thin tape to measure circumference of tires. Best and most accurate way to get the actual size or difference in size between two tires. Then you can add or subtract air as necessary.
 
 John

[Seldom Seen Slim]

I vaguely remember the formula for circumference as being pi times DIAMETER, not radius, so measuring from ground to center of axle would only give half the distance around the outside of the tire, right?
 
 Us bike folks don't have to match up rear tires, but I use the circumferential tape measure method anyway -- tells me the effective gear ratio (change) when I'm changing sprockets and tires.

[Rex Schimmer]

Ron,
 This is one of the differences between drag racing and landspeed racing. In drag racing they are concerned with the radius from the axel centerline to the ground and when you run a drag slick at 7 psi that distance is alot less than the diameter divided by two and this makes a difference when it comes to acceleration. In landspeed racing the important dimension is the actual circumferance, as land speed tires are typically ran at 60-100 psi or more, there is very little "flatening" of the tread when the tire is on the ground. The circumferance is important because if the the tire is not slipping then for every rotation of the tire the car advances the length of the circumferance. I normally use the old formula for speed that is based upon the tire diameter.
 
  speed= tire diameter X rpm/336 X gear ratio
 
 So I use a machinest "pie" tape which allows you to run the tape around the tire and then gives you the diameter. It is accurate to within about .010 inches.  The real "challenge" here is tire growth, top fuel drag slicks grow 8-10 inches in diameter at 300 mph and I am sure that landspeed tires also grow but my guess is much less, probably less than one inch.  As far as what the NASCAR and others do by changing car handling with tire pressure changes,there are a lot of ideas as to what the real way that tire pressure changes effect car handling. One is that the pressure change changes the spring rate of the tire and therefore is like changing the suspension spring. This "spring rate" change probably has a very small effect as it is quite small and requires a very rigid chassis to really have any effect. Mark Ortiz, the "Consultant" on suspension in "Racecar Engineering" magazine says that the real effect is that the tire patch changes size and this makes the real traction change. If you happen to be running a landspeed car that has a very narrow track, then these changes have a smaller effect than a  car with a full width track.

[Justin]

Rex, I have been trying to find a tire guage that will measure the type of pressures that you mentioned, could you tell me where they are available?

[Bob Drury]

Justin, try speedway motors, or most any of the better digital gauges.  Just a quick reminder, although the LSR rated tires call for 90 psi inflation, I am not sure if guys running the
 Goodyear frontrunners,etc., should be running that kind of pressure.

[RichFox]

Justin: Or you could just get a good pressure gage and screw a tire valve line to it. Maybe look in the yellow pages under Pressure Gages.

[Ken Walkey]

Believe it or not, Pep Boys has a very good hi-pres air gage. Goodyear calls for 70psi in most of their LSR tires, 60 in the 2ply frontrunners.

[wmtsmith]

Tire growth---MT's at 90psi grew 1.3" at 270mph

[Ron Gibson]

Rex
  I understand what you are saying about drag tires but LSR tires have little growth per Sparky's post.
 My geometry (which is old and wrong per Slim's post LOL) tells me that axle center X 2 X Pi would be the true roll out of the tire, regardless of pressure or contact patch. I can visualize that more air in one to make them measure the same circumference might increase the axle measurement on that side to where the small tire becomes the larger due to squash. My math (which is also old) says 1/4 inch larger at 7000 rpm w/2.75 gears means 53 feet difference between the tires per minute. That much could lead to handling problems depending on axle width as you said. No dog in the hunt, just work on others but curious and trying understand.

[JackD]

Tires that start out the same size by every measurement will each change differently at speed depending on the load they see.
 Because you have them tied together with a spool only makes them turn at the same speed.
 The static weight, engine torque, and surface variables all contribute to the mix.
 Get them as close as you can but realize it is like splitting hairs in a comb over, it's ok if you get the girl.

[Justin]

Thanks for the info.

[Rex Schimmer]

Ron,
 Your math is right and the tire roll out is the important meausurement so I think I would sort tires until you found the same rollout on two tires at the same pressure. Probably a small difference in the tire pressure to get the same rollout would probably not have a big effect on handling.
 As far as a 100 psi tire gage I just bought a standard tire gage with a 2-1/4 0-60 gage and bought a new 0-100 gage a the local hydraulics store, about $12.00.
 Rex